diff -r 000000000000 -r 6474c204b198 js/src/jit-test/tests/collections/Map-constructor-duplicates.js
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/js/src/jit-test/tests/collections/Map-constructor-duplicates.js	Wed Dec 31 06:09:35 2014 +0100
@@ -0,0 +1,8 @@
+// When the argument to Map contains a key multiple times, the last value is retained.
+
+var arg = [["zero", 7], ["one", 1], ["two", 4], ["zero", 8], ["two", 2], ["zero", 0]];
+var m = Map(arg);
+assertEq(m.get("zero"), 0);
+assertEq(m.get("one"), 1);
+assertEq(m.get("two"), 2);
+assertEq(m.size, 3);